1. State of the Art:
Optical mark and optical character readers which employ line scan cameras having an array of photosensitive elements develop analog output signals on a line by line basis of a scanned document which provide an indication of the presence or absence of characters or marks on the basis of light reflection from or transmission through the document.
The black/white signature of a scan line, or sweep of line scan camera, across a document, such as a student's test answer sheet, a voting ballot, an order entry form, etc., produces an analog video output signal from the camera. The analog signal is applied to a video normalizer circuit to initially provide a black level compensation and determine the gray level of the discrete picture elements (pixel) of the discrete portions of the analog signal corresponding to the individual photosensitive elements of the line scan camera. The gray level analog output signals are supplied to an analog to digital converter which generates a two-bit output identifying the gray level of each picture element as one of several levels, i.e., 00, 01, 10, and 11, which is subsequently analyzed to determine if the gray level corresponds to an acceptable valid mark or character on the document, or is indicative of the absence of a mark or the presence of a mark which is not sufficiently dark to be recognized as a valid mark. This multi-level mark, or character, density discrimination provided by the two-bit output of the analog to digital converter permits electronic differentiation between a smudge, an inadvertent pencil mark, and a legitimate mark or character appearing on the document.
The utilization of a line scan camera and digital signal processing circuitry for deriving intelligence from a document is described in detail in pending U.S. Application Ser. No. 000,457, filed Jan. 2, 1979, entitled, "Optical Reading System" by J. V. McMillan and D. W. Schroeder, which is assigned to the assignee of the present invention and incorporated herein by reference.
In order for the typical state of the art systems to operate effectively, the system recognizes marks appearing in data entry mark locations only when the marks are preceded by a special printed document mark called a scan mark. The scan marks are used for document verification and registration. This feature precludes the use of the scanner from data sheets where the pre-printed data entry mark locations on the document are located in advance of the printed scan mark.
In order to provide a desired flexibility for such a document scanning system, it is desirable to provide a delay technique whereby the digitized video output signal from the analog to digital converter is delayed by a prescribed number of lines prior to processing. The number of line delays effected by the delay circuit corresponds to the sum of available lines of entry marks on a document which precede the preprinted scan mark and the number of camera scan lines of the preprinted scan mark employed to verify the presence of the preprinted scan mark.
Thus, on a document format which includes a data entry line corresponding to two scan lines of the camera prior to the presence of a preprinted scan mark, and the system employs two scan lines of the scan mark in order to verify the presence of the scan mark, a suitable delay circuit would correspond to a four line delay.